This is a continuation in part of my copending patent application Ser. No. 490,355 filed Aug. 13, 1974, now abandoned.
In 1968 Peter Glaser introduced his concept for an orbiting solar power station which converts solar radiation into microwave radiation which is beamed continuously to earth where it is reconverted into electric power for distribution. (See, "Power from the Sun: Its Future", Science, Vol. 162, pp. 857-861, 1968.) Unfortunately, since the solar constant (the amount of solar radiation falling on 1 square meter of area perpendicular to the radiation and at a distance of one astronomical unit) is only 1.34 KW/m.sup.2 and since the best solar cells convert solar radiation into electric power with an efficiency of only about 12%, it requires an enormous array of solar cells to produce high electric power levels on the order of several GW (1 GW = 10.sup.9 watts). In order to generate 1 GW of electric power the solar array must be 6.2 .times. 10.sup.6 m.sup.2 which is equivalent to a square array 2.5 kilometers on end. But an array this large is unavoidable if the electric power is to be used to generate continuous electromagnetic radiation for continuous wireless power transmission.
There are other important uses for beamed electromagnetic radiation which does not require continuous power transmission. In my U.S. Pat. No. 3,825,211 entitled LASER ROCKET, filed June 19, 1972, I show how a rocket vehicle can be propelled by converting the radiant power in a laser beam, that is intercepted by the vehicle, into propulsive thrust. This allows the relatively massive energy generating mechanism to be separated from the vehicle which results in increased vehicle performance. Since the thrusting maneuvers last only a few minutes and are separated by relatively long periods of time (usually 24 hours) the laser generator need not remain in continuous operation. A similar rocket propulsion system is described in my copending U.S. patent application Ser. No. 343,197 entitled MICROWAVE POWERED REUSABLE ORBITING SPACE TUG, filed Mar. 21, 1973, now U.S. Pat. No. 3,891,160. Therefore, the relatively long time intervals between successive power transmissions can be utilized for accumulating energy in an energy storage system by relatively low input power and extracting it at a much higher rate over a much shorter time period during power transmission.
The fundamental differences between prior art orbiting solar power stations and this solar power station is the addition of a high capacity, rechargeable, energy storage system for accumulating solar energy over relatively long time intervals and retransmitting it later over short time intervals at higher powers. Thus, the design of a light weight, high flux, high capacity, rechargeable energy storage system will be an important part of this patent.
The most important characteristics of large energy storage systems are: high energy storage capacity, high efficiency, high energy density and high power density. Energy density is defined as the total energy storage capacity of the system divided by its total mass E/M and power density is defined as the highest power P that the system can generate during a discharge divided by its total mass P/M. Due to the extremely high transportation cost of launching payloads into orbit, a space based energy storage system should have the highest possible energy density. In a recent patent entitled "Inertial Energy Storage Apparatus and System for Utilizing the Same," U.S. Pat. No. 3,683,216 by Richard Post, there is described a flywheel energy storage system that is designed to operate close to the maximum possible energy density limit. This limit (which is the same for all kinetic energy containment systems such as flywheels or the containment of hot gases) can be derived from the virial theorem (see Elementary Plasma Physics, Interscience Publishers, 1963 by Longmire). In particular, if M denotes the structural mass with density .pi. that is used to contain an amount of kinetic energy E with a uniaxial stress .sigma., then ##EQU1## (MKS units are employed throughout this paper). In the case of a spinning flywheel, M refers to the mass of the flywheel and E denotes its kinetic energy. This energy is given by the equation E = 1/2 I.omega..sup.2 where I denotes the flywheel's moment of inertia about its spin axis and .omega. is its angular velocity. An optimally designed flywheel energy storage system, therefore, has a maximum possible energy density given by ##EQU2## where .pi. and .sigma. denote the flywheel's density and tensile strength respectively. When compared to other rechargeable energy storage systems, such as batteries, fuel cells, compression of gases, pumped-hydro, elastic deformation (steel springs) etc. the Post flywheel system has energy densities and charge and discharge rates many times greater.
Recent advances in the field of superconductivity have now opened the way to a totally new concept of energy storage that does not involve kinetic energy. This concept utilizes the inductive energy stored in a magnetic field. Until relatively recently, high magnetic fields could only be obtained by passing large currents through the coils of electromagnets. These coils, being normal conductors, such as copper or aluminium, had inherent resistance. The power P (expended in the form of heat) that is required to maintain the magnetic field is given by the equation P = i.sup.2 R where i and R denote the current and resistance of the coil. Since high field electromagnets require very high currents, the power consumed by electrical resistance in high field electromagnets is enormous.
The development of superconducting materials (namely Type II superconductors) however, now makes possible superconducting magnets having coils of zero resistance. A current passing through the coil of a superconducting magnet has no resistance and will remain constant without loss of power. (See "Large-Scale Applications of Superconducting Coils", Proceedings of the IEEE, Vol. 61, No. 1, January, 1973, by Z. Stickle and R. Thome.) The applications of superconducting magnets as depositories of electrical energy is possible by utilizing the coil's self inductance. If a superconducting coil has a self-induction L, the magnetic field has an inductive energy given by the equation EQU E = 1/2 L i.sup.2 ( 2)
Consequently, superconducting magnets can be used for storing electrical energy without loss. Devices known as flux pumps and homopolar generators are used to transfer current in and out of a superconducting coil which becomes the means by which energy is fed into and out of the magnet. (See McGraw-Hill Encyclopedia of Science and Technology, Vol. 13, 1971, p. 306, and "Superconductive Energy Storage for Power Systems", IEEE Transactions on Magnetics, Vol. MAG-8, No. 3, p. 701, September 1972 by R. Boom and H. Peterson.) The result is a magnetic energy storage system with no moving parts that can go through an unlimited number of charge and discharge cycles without ever wearing out and that has an overall in-out operating efficiency very close to 100%. It should be pointed out, however, that since the superconducting coil has to be kept at very low cryogenic temperatures, a refrigeration system will be necessary and this system will require the expenditure of some power. But this energy loss will be very small compared to the energy storage capacity. Moreover, if the storage system is operated in an environment that is very cold to begin with, such as in outer space and shielded from the sun's rays, the refrigeration system will require very little power.
For inductive energy storage systems, the virial theorem requires only half of the restraining structural mass needed for mechanical systems based on kinetic energy confinement. For inductive systems ##EQU3## In this inequality, the mass M refers to the conductor's mass (i.e., coil mass) which generates the magnetic field. Consequently, for magnetic energy storage systems, the maximum energy density is given by the equation EQU E/M = .sigma./.pi. (3)
where E refers to the magnetic inductive energy given by equation (2). Thus, in theory, a superconducting magnetic energy storage system offers the possibility of obtaining energy densities twice those that are possible with flywheel systems.
As in the case of flywheel systems, the geometrical shape of the coil is an important consideration and can greatly affect the value of E/M given in the above equation for magnetic energy storage systems. It will be shown that coils in the form of thin donuts will give E/M values fairly close to the virial limit given above.
Donut-shaped coils are called dipoles. They are among the few coil shapes that are entirely self-supporting when energized with a current. Thus, the dipole coil also supplies the required structural material that is required to restrain the very large magnetic forces that tend to break apart the coil. These J .times. B Lorentz forces arise when current elements in the coil, with current density J, interact with the resulting magnetic field B. They tend to expand the dipole radially outward from its center (which results in a large tension force) and secondary forces tending to compress the ring of the dipole inward toward its minor axis.
Since thick dipole magnets are difficult to cool and thin dipole magnets require large areas for their installation, most prior art superconducting magentic energy storage systems use solonodial or toriodial coil shapes. But these shapes are not self-supporting and require a great amount of structural material (usually steel) to prevent their collapse. This added material, however greatly increases the system's mass which results in a decrease in energy density E/M. However, for large stationary, earth based systems, energy density is not considered important. But for orbiting systems the energy density will be very important. Thus, one of the major goals of this patent is to present a light weight, high energy density, superconducting magnetic energy storage system that is entirely self-supporting. Hence, in order to take advantage of the self-supporting characteristics of dipole coils the magnetic energy storage system presented here will be based on a dipole coil configuration.
A dipole superconducting self-supporting magnetic energy storage system offers the possibility of obtaining a rechargeable energy storage system with an in-out efficiency of nearly 100%. Moreover, it will be shown that the system also offers the theoretical possibility of doubling the energy density of the most efficient flywheel systems. Consequently, for equal amounts of stored energy, a dipole, self-supporting magnetic energy storage system offers the possibility of reducing the required structural mass to almost one-half that required for the most efficient flywheel systems. Since the system will have to be transported to an orbit high above the earth, which will be a very costly process, this possibility for significantly reducing structural mass without reducing the amount of stored energy capability is an important consideration. It should also be pointed out that the vacuum of space is an ideal environment for a superconducting magnetic energy storage system. The system could be easily shielded from the sun's rays so that it will be relatively easy to maintain the required cryogenic temperatures. Furthermore, the resulting magnetic field does not have to be shielded as they are for earth bound systems and the over all dimensions of the system could be very large. Indeed, space may well be the optimum environment for large superconducting magnetic energy storage systems. Furthermore, since the power density capability of magnetic energy storage systems are tremendous-where even relatively small systems have the ability of discharging at several GW (1GW = 10.sup.9 watts) - the power released from a magnetic energy storage system will be more than enough to power any laser or microwave generator for wireless power transmission.
Thus, according to principles of this invention, a relatively small array of solar cells is maintained in an orientation perpendicular to the sun's rays. The resulting current is fed into the coil of a superconducting magnet which increases its magnetic field. The input energy is thereby accumulated and stored inductively in the coil's magnetic field. When this energy is released, it will reappear as electric current, only at power levels many times higher than that generated by the solar array. This power is then used to energize a high power laser or microwave transmitter for intermittent wireless power transmission.
It is estimated that 72% of the total installation cost of Glaser's orbiting solar power station (that transmits continuous power) will be expended just in transporting the enormous solar array from the earth's surface to orbit. (See, "Solar Power Via Satellite", Astronautics and Aeronautics, Vol. 11, No. 8, August 1973, pp. 60-68, by Peter Glaser.) The system presented in this patent will allow wireless power transmission at the same levels as Glaser's station but with a much smaller solar array. This will result in a significant reduction in cost. The only penalty will be in the total amount of energy transmitted over long time periods. The flux, however, can remain unchanged and could even be greater than that contemplated by Glaser. For the applications envisioned in this patent, continuous power transmission is not needed.